A peak-to-average power ratio (PAR) of wireless transmit signals is rather high in current radio systems and it is highly desirable to find ways to reduce it without reducing the radio link performance in order to improve the efficiency of the transmitter equipment. The efficiency of a power amplifier of a base station transmitter in a radio system depends on a peak-to-average power ratio of the transmitted signal. The lower the PAR, the better the power amplifier efficiency can be.
An often-used method for reducing PAR is clipping in which high amplitude values of the transmitted signal are clipped and an error is created in the transmitted signal. The amount of the created error is limited by such as error vector magnitude (EVM) and PCDE (peak code domain error) requirements as well as by the requirements for spectrum emission masks (SEM). Several types of iterative clippers have been considered for OFDM (orthogonal frequency division multiplexing) clipping, for example, in J. Tellado: “Peak-to-Average Power Reduction for Multicarrier modulation”, Ph.D. thesis, Stanford University, September 1999. Further, an example of a WCDMA (wideband code division multiple access) clipper algorithm solution is described in US 2003/0219079.
One problem with the existing clipper solutions is that as the PAR value of the transmitted signal is reduced, the created error value increases at the same time. Thus, there is a need to find new iterative clippers that could perform better than the existing algorithms and with reasonable complexity.